The present invention relates generally to wiper blades for motor vehicles and air vehicles.
Wiper blades of the above mentioned type are subjected to high mechanical loads by dust, insect residues, or other hard dirt components which make the glass surface to be cleaned rough. On the other hand, they are subjected to mechanical loads by the tilting which always alternates during the wiping process in its direction, and in the movable condition because of the pressure against the glass pane. In addition to the mechanical loads, also chemical actions or environmental actions affect the wiper blades, for example ozone, oil as well as alcohol and/or detergents in the window washing fluid.
Wiper blades are produced as a rule continuously by extrusion or coextrusion of cross-linked polymers or polymer mixtures. The extruded strands are cross-linked to elastomeric profiles and cut into the required length. The wiper blades have a wiper head (also known as a base part or a back), with which the wiper blade is mounted in a bracket system of the window wipers. Holes or recesses are provided in the wiper blade for one or several spring rails of an elastic metal, which stabilize the wiper blade and must provide a maximum uniform pressure over the total length of the wiper blade.
At the side facing the glass pane the wiper head has a tilting web which is known as a neck, a hinge or a web. The wiper head is connected with a wiper lip which during operation of the window wiper acts as a functional part of the wiper blade which holds the glass pane free from water. The tilting web is the part of the wiper blade which is mechanically loaded the most. Depending on its height and design of the opposite surfaces of the wiper head and the wiper lip, the wiper lip in the movable condition can deviate with its main transverse axis (identified in the drawing with reference numeral 4) by up to 45.degree. from the perpendicular to the glass pane. In the operational condition the lip runs behind the head, whereby during each directional change a tilting movement is required. The main transverse axis changes correspondingly its position during the operation by up to substantially .+-.-45.degree..
The wiper lips on the one hand can be adjusted to the geometry of at least curved glass pane or in other words must be flexible. On the other hand they have to be sufficiently hard to provide mechanical removal of dirt particles deposited on the glass pane. Finally, they must be wear-resistant, so that over a long time they provide high cleaning quality without forming stripes.
Wiper blades in accordance with the prior art have all parts composed of a single elastomeric material. This is very convenient for manufacture. However, this does not take into consideration different requirements for different parts of the wiper blades. When for example the material is relatively soft and highly elastic, it satisfies important requirements with respect to the tilting web. However, it suits very little for the head, since during the operation not seldom it turns out of the claws which hold the wiper blade in the metal bracket. When the material to the contrary is harder and less elastic, it is well suitable for the head, but less suitable for the mechanical loads to which the tilting web is constantly subjected.
Wiper blades only from one material, namely natural rubber or a mixture of natural rubber and chloroprene rubber or a rubber of diene type, butadiene, butadiene rubber and styrol-butadiene rubber are disclosed for example in the German patent document DE-C2 35 27 093. Natural rubber and other rubbers of diene type contain olefin double bindings and are sensitive to ozone. By treatment with a hypochlorite (chlorinating) the surface of the wiper blade is hardened. This is advantageous for the wiper lip whose wear resistance is increased. However, a compromise has to be maintained when a complete chlorination of the surface is performed to provide wear resistance, since it makes the surface of the lip less flexible so that it does not sufficiently adjust to the glass pane. For a tilting web of natural rubber and/or other rubbers of diene type the chlorination is damaging since it provides a permanent deformation under the action of pressure. Such an action is applied to the wiper blade in a movable condition and in operation since it is continuously pressed against the glass pane.
The wiper pane in accordance with the invention disclosed in the German patent document DE-C2 35 27 093 is composed substantially only of one material, namely EPDM rubber, a mixed polymer of ethylene, propylene and diene which is cross-linked with peroxides and is resistant against ozone. The wiper blade at the lower end of the lip which faces the glass pane and in the upper peripheral zones of the head has small segments of secondary quantity of diene rubber. EPDM has a high friction coefficient which makes difficult insertion in particular of the inwardly located spring strips and makes necessary a surface coating of the springs strips or the use of the sliding-assisting means such as molybdenum sulfide.
Wiper blades of different, coextruded materials are known, in which in contrast to the German patent document DE-C2 35 27 093 the different materials are provided in comparable quantities. In the European patent document EP-A1 0 625 452 a wiper blade is disclosed which is composed at least of two different materials. In one described embodiment the head and the tilting web are composed of the same material, while a lip which is composed of a different material surrounds a reinforcement in which the small tilting web runs. This embodiment have the above mentioned disadvantage that the material is not completely right for different requirements applied to the head and to the web.